Femtosecond Laser-Structured Underwater "Superpolymphobic" Surfaces.

In this work, the surfaces that repel liquid polydimethylsiloxane (PDMS) droplets in water were created by femtosecond laser treatment. We define this superwetting phenomenon as underwater "superpolymphobicity". The resultant underwater superpolymphobic silicon surface shows a contact angle of 159 ± 1° and a sliding angle of 1.5 ± 0.5° to liquid PDMS droplets in water. This underwater superpolymphobicity can be achieved on a wide range of hydrophilic materials, including semiconductors, glass, and metals. The adhesion between the liquid polymer and a solid substrate is effectively prevented by the underwater superpolymphobic microstructures. The underwater superpolymphobicity will have a great significance in designing the adhesion between the polymer and a solid substrate, controlling the shape of the cured polymer materials, as well as nearly all the applications based on the polymer materials.

How To Obtain Six Different Superwettabilities on a Same Microstructured Pattern: Relationship between Various Superwettabilities in Different Solid/Liquid/Gas Systems.

A range of different superwettabilities were obtained on femtosecond laser-structured Al surfaces. The formation mechanism of each superwetting state is discussed in this paper. It is revealed that the underwater oil droplet and bubble wettabilities of a solid surface have a close relationship with its water wettability. The laser-induced hierarchical microstructures showed superhydrophilicity in air but showed superoleophobicity/superaerophobicity after immersion in water. When such microstructures were further modified with a low-surface-energy monolayer, the wettability of the resultant surface would turn to superhydrophobicity with ultralow water adhesion in air and superoleophilicity/superaerophilicity in water. The understanding of the relationship among the above-mentioned six different superwettabilities is highly important in the design of various superwetting microstructures, transforming the structures from one superwetting state to another state and better using the artificial superwetting materials.

Effective approach to strengthen plasmon resonance localized on top surfaces of Ag nanoparticles and application in surface-enhanced Raman spectroscopy.

The spatial distribution of localized surface plasmon resonance (LSPR) plays a key role in many plasmonic applications. Based on the thermal stability of alumina templates, this work reports a novel approach to manipulate the distribution of LSPR and exhibits its significance for an important plasmonic application, the surface-enhanced Raman spectroscopy (SERS). A suitable thermal annealing sharpens the edges in top surfaces (far from the substrates) of Ag nanoparticles, which significantly strengthens the distal mode (DM) with the LSPR excited on the top surfaces. Because the top surface is the major place to adsorb probe molecules, this manipulation greatly improves the detection sensitivity of SERS. Our research provides a new way to improve the sensitivity of SERS, which also indicates that great care has to be taken on special LSPR mode which is largely responsible for a certain plasmonic application (e.g., the DM for SERS although it is not the major mode).

Photocatalytic bactericidal mechanism of nanoscale TiO2 films on Escherichia coli.

Two kinds of nanoscale TiO2 films were prepared by magnetron sputtering and screen printing methods, respectively. Results show that both phase composition and specific surface area of the film affect the photocatalytic bactericidal efficiency. Time-series in situ atomic force microscopy (AFM) observation were further used to characterize the cellular responses of Escherichia coli (E. coli) in photocatalytic process. Some nanosized patches were found on the bacterial surface in the forepart of photocatalytic reaction. It suggested that the photocatalytic attack induced the self-protection of bacteria at first. Subsequently, some cracks on the surface and the enlargement of cell body indicated that the cell wall was damaged and lost its structure supporting function, and it eventually led to the death of bacteria.

Multipronged heat-exchanger based on femtosecond laser-nano/microstructured Aluminum for thermoelectric heat scavengers.

Femtosecond (fs) laser processing can significantly alter the optical, thermal, mechanical, and electrical properties of materials. Here, we show that fs-laser processing transforms aluminum (Al) to a highly efficient and multipronged heat exchanger. By optimizing the formed surface nano- and microstructures, we increase the Al emissivity and surface area by 700% and 300%, respectively. Accordingly, we show that fs-laser treated Al (fs-Al) increases the radiative and convective cooling power of fs-Al by 2100% and 300%, respectively, at 200 °C. As a direct application, we use fs-Al as a heat sink for a thermoelectric generator (TEG) and demonstrate a 280% increase in the TEG output power compared to a TEG with an untreated Al heat exchanger at 200 °C. The multipronged enhancement in fs-Al heat exchange properties lead to an increase in the TEG output power over a wide temperature ( T ) range ( T > 50 °C ). Conversely, a simple radiative cooling heat exchanger increases the TEG output power within a limited temperature range ( T > 150 °C ) . We investigate the laser processing parameters necessary to maximize the spectral emissivity and surface area of fs-Al. Fs-Al promises to be a widely used and compact heat exchanger for passive cooling of computers and data centers as well as to increase the efficiency of TEGs incorporated in sensors and handheld electronics.

Robust mold fabricated by femtosecond laser pulses for continuous thermal imprinting of superhydrophobic surfaces.

Superhydrophobic surfaces rely on a large number of surface micro/nano structures to increase the roughness of a material. Producing such structures is possible through a multitude of relatively slow methods; however, economic and large scale production of superhydrophobic surfaces require using a fast process on a cheap substrate. Here, we used femtosecond laser processing to fabricate micro and nanostructures on tungsten carbide that we use as a mold to thermally imprint polypropylene sheets. The fabricated tungsten carbide mold was used to imprint more than twenty superhydrophobic polypropylene sheets before mold contamination reduces the surface contact angle below 150°.Using Toluene solution, the mold is subsequently capable of being cleaned of contamination from polypropylene residue and reused for further imprinting. Ninety thermoplastic imprints were conducted using a single tungsten carbide mold with only minimal structural degradation apparent on the micro/nano structured surface.

Femtosecond-Laser-Produced Underwater "Superpolymphobic" Nanorippled Surfaces: Repelling Liquid Polymers in Water for Applications of Controlling Polymer Shape and Adhesion.

A femtosecond (fs)-laser-processed surface that repels liquid polymer in water is reported in this paper. We define this phenomenon as the "superpolymphobicity". Three-level microstructures (including microgrooves, micromountains/microholes between the microgrooves, and nanoripples on the whole surface) were directly created on the stainless steel surface via fs laser processing. A liquid polydimethylsiloxane (PDMS) droplet on the textured surface had the contact angle of 156 ± 3° and contact angle hysteresis less than 4° in water, indicating excellent underwater superpolymphobicity of the fs-laser-induced hierarchical microstructures. The contact between the resultant superhydrophilic hierarchical microstructures and the submerged liquid PDMS droplet is verified at the underwater Cassie state. The underwater superpolymphobicity enables to design the shape of cured PDMS and selectively avoid the adhesion at the PDMS/substrate interface, different from the previously reported superwettabilities. As the examples, the microlens array and microfluidics system were prepared based on the laser-induced underwater superpolymphobic microstructures.

Highly Floatable Superhydrophobic Metallic Assembly for Aquatic Applications.

Water-repellent superhydrophobic (SH) surfaces promise a wide range of applications, from increased buoyancy to drag reduction, but their practical use is limited. This comes from the fact that an SH surface will start to lose its efficiency once it is forced into water or damaged by mechanical abrasion. Here, we circumvent these two most challenging obstacles and demonstrate a highly floatable multifaced SH metallic assembly inspired by the diving bell spiders and fire ant assemblies. We study and optimize, both theoretically and experimentally, the floating properties of the design. The assembly shows an unprecedented floating ability; it can float back to the surface even after being forced to submerge under water for months. More strikingly, the assembly maintains its floating ability even after severe damage and piercing in stark contrast to conventional watercrafts and aquatic devices. The potential use of the SH floating metallic assembly ranges from floating devices and electronic equipment protection to highly floatable ships and vessels.

Microfluidic Channels Fabrication Based on Underwater Superpolymphobic Microgrooves Produced by Femtosecond Laser Direct Writing.

A strategy is proposed here to fabricate microfluidic channels based on underwater superpolymphobic microgrooves with nanoscale rough surface structure on glass surface produced by femtosecond (fs) laser processing. The fs laser-induced micro/nanostructure on glass surface can repel liquid polydimethylsiloxane (PDMS) underwater, with the contact angle (CA) of 155.5 ± 2.5° and CA hysteresis of 2.7 ± 1.5° to a liquid PDMS droplet. Such a phenomenon is defined as the underwater "superpolymphobicity". Microchannels as well as microfluidic systems are easily prepared and formed between the underwater superpolymphobic microgroove-textured glass substrate and the cured PDMS layer. Because the tracks of the laser scanning lines are programmable, arbitrary-shaped microchannels and complex microfluidic systems can be potentially designed and prepared through fs laser direct writing technology. The concept of "underwater superpolymphobicity" presented here offers us a new strategy for selectively avoiding the adhesion at the polymer/substrate interface and controlling the shape of cured polymers; none of these applications can find analogues in previously reported superwetting materials.

Substrate-Independent, Fast, and Reversible Switching between Underwater Superaerophobicity and Aerophilicity on the Femtosecond Laser-Induced Superhydrophobic Surfaces for Selectively Repelling or Capturing Bubbles in Water.

In this paper, the reversible switching between underwater (super-) aerophilicity and superaerophobicity was achieved on various femtosecond (fs) laser-induced superhydrophobic surfaces. A range of materials including Al, stainless steel, Cu, Ni, Si, poly(tetrafluoroethylene), and polydimethylsiloxane were first transformed to superhydrophobic after the formation of surface microstructures through fs laser treatment. These surfaces showed (super-) aerophilicity when immersed in water. In contrast, if the surface was prewetted with ethanol and then dipped into water, the surfaces showed superaerophobicity in water. The underwater aerophilicity of the superhydrophobic substrates could easily recover by drying. The switching between the underwater aerophilicity and superaerophobicity can be fast repeated many cycles and is substrate-independent in stark contrast to common wettability-switchable surfaces based on stimuli-responsive chemistry. Therefore, the as-prepared superhydrophobic surfaces can capture or repel air bubbles in water by selectively switching between underwater superaerophobicity and aerophilicity. Finally, we demonstrated that the underwater bubbles could pass through an underwater aerophilic porous sheet but were intercepted by an underwater superaerophobic porous sheet. The selective passage of the underwater bubbles was achieved by the reversible switching between the underwater aerophilicity and superaerophobicity. We believe that this substrate-independent and fast method of switching air wettability has important applications in controlling air behavior in water.

Fluorescence enhanced lab-on-a-chip patterned using a hybrid technique of femtosecond laser direct writing and anodized aluminum oxide porous nanostructuring.

In this paper, we demonstrate a simple yet effective hybrid method to fabricate lab-on-a-chip devices on aluminum (Al) foil. Instead of using conventional photoresists and lithography methods, an array of square units is first produced by femtosecond laser direct writing, followed by generating highly ordered anodized aluminum oxide (AAO) nanoporous structures within each unit. The AAO treated area becomes hydrophilic. Next, we functionalize the surrounding area outside the square units to superhydrophobic by electrochemical deposition and further chemical modification. This hydrophilic and hydrophobic pattern allows us to confine the liquid samples to be detected within the hydrophilic AAO detection area. We use rhodamine 6G (R6G) as a probe, and obtain a fluorescence intensity enhancement from R6G by 70 times over a flat surface. This leads to the detection sensitivity of R6G molecules to a concentration as low as 10-17 mol L-1. By mixing R6G with RhB molecules, the fluorescence emission bands shift significantly due to the addition of RhB molecules, showing a significantly improved spectral resolution compared to traditional fluorescence spectrometers for liquid samples. This phenomenon can be attributed to the energy transfer between R6G and RhB under laser excitation, which was enhanced by the AAO nanostructures. The array-based LOC device demonstrated in this paper is simple and convenient to fabricate, has low sample consumption and dramatically enhances the fluorescence yield with improved spectral resolution.

Superhydrophobic Al Surfaces with Properties of Anticorrosion and Reparability.

Aluminum (Al) is one of the most widely used metals for industry and household applications, but its longevity is limited by its tendency for corrosion. In this work, we report a facile method to fabricate superhydrophobic Al surfaces that have excellent anti-corrosion effect. The surface is obtained by etching Al in CuCl2 solution to form the micro-nano-pit surface texture followed by lowering its surface energy in an aqueous ethanol solution of stearic acid. The superhydrophobic Al surfaces show water contact angles as high as 165°. Electrochemical tests demonstrate that the corrosion rate of the Al surface drops by 94.5% after the superhydrophobic modification (corrosion current density lowers from 1.11 × 10-4 to 6.10 × 10-6 A cm-2). We also show that the superhydrophobic surface will protect the Al from corrosion even under a very harsh environment. In addition, our method is scalable and the superhydrophobic surfaces exhibit excellent flexible and reparable properties. This anti-corrosive superhydrophobic Al surface will prolong Al in its broad usage.

Manipulation of charge transfer and transport in plasmonic-ferroelectric hybrids for photoelectrochemical applications.

Utilizing plasmonic nanostructures for efficient and flexible conversion of solar energy into electricity or fuel presents a new paradigm in photovoltaics and photoelectrochemistry research. In a conventional photoelectrochemical cell, consisting of a plasmonic structure in contact with a semiconductor, the type of photoelectrochemical reaction is determined by the band bending at the semiconductor/electrolyte interface. The nature of the reaction is thus hard to tune. Here instead of using a semiconductor, we employed a ferroelectric material, Pb(Zr,Ti)O3 (PZT). By depositing gold nanoparticle arrays and PZT films on ITO substrates, and studying the photocurrent as well as the femtosecond transient absorbance in different configurations, we demonstrate an effective charge transfer between the nanoparticle array and PZT. Most importantly, we show that the photocurrent can be tuned by nearly an order of magnitude when changing the ferroelectric polarization in PZT, demonstrating a versatile and tunable system for energy harvesting.

Highly Controllable Surface Plasmon Resonance Property by Heights of Ordered Nanoparticle Arrays Fabricated via a Nonlithographic Route.

Perfectly ordered nanoparticle arrays are fabricated on large-area substrates (>cm(2)) via a cost-effective nonlithographic route. Different surface plasmon resonance (SPR) modes focus consequently on their own positions due to the identical shape and uniform size and distance of these plasmonic metallic nanoparticles (Ag and Au). On the basis of this and FDTD (finite-difference time-domain) simulation, this work reveals the variation of all SPR parameters (position, intensity, width, and mode) with nanoparticle heights, which demonstrates that the effect of heights are different in various stages. On increasing the heights, the major dipole SPR mode precisely blue-shifts from the near-infrared to the visible region with intensity strengthening, a peak narrowing effect, and multipole modes excitation in the UV-vis range. The intensity of multipole modes can be manipulated to be equal to or even greater than the major dipole SPR mode. After coating conformal TiO2 shells on these nanoparticle arrays by atomic layer deposition, the strengthening of the SPR modes with increasing the heights results in the multiplying of the photocurrent (from ∼2.5 to a maximum 90 µA cm(-2)) in this plasmonic-metal-semiconductor-incorporated system. This simple but effective adjustment for all SPR parameters provides guidance for the future design of plasmonic metallic nanostructures, which is significant for SPR applications.

Highly Reproducible and Sensitive SERS Substrates with Ag Inter-Nanoparticle Gaps of 5 nm Fabricated by Ultrathin Aluminum Mask Technique.

Applicable surface enhanced Raman scattering (SERS) active substrates require high enhancement factor (EF), excellent spatial reproducibility, and low-cost fabrication method on a large area. Although several SERS substrates with high EF and relative standard deviation (RSD) of signal less than 5% were reported, reliable fabrication for large area SERS substrates with both high sensitivity and high reproducibility via low-cost routes remains a challenge. Here, we report a facile and cost-effective fabrication process for large-scale SERS substrate with Ag inter-nanoparticle (NP) gaps of 5 nm based on ultrathin alumina mask (UTAM) surface pattern technique. Such closely packed Ag NP arrays with high density of electromagnetic field enhancement ("hot spots") on large area exhibit high SERS activity and excellent reproducibility, simultaneously. Rhodamine 6G molecules with concentration of 1 × 10(-7) M are used to determine the SERS performance, and an EF of ∼10(9) is obtained. It should be noted that we obtain RSDs about 2% from 10 random spots on an area of 1 cm(2), which implies the highly reproducible signals. Finite-difference time-domain simulations further suggest that the enhanced electric field originates from the narrow gap, which agrees well with the experimental results. The low value of RSD and the high EF of SERS signals indicate that the as-prepared substrate may be promising for highly sensitive and uniform SERS detection.

Facile Transferring of Wafer-Scale Ultrathin Alumina Membranes onto Substrates for Nanostructure Patterning.

Ordered nanostructure arrays have attracted intensive attention because of their various applications. However, it is still a great challenge to achieve ordered nanostructure patterning over a large area (such as wafer-scale) by a technique that allows high throughput, large pattern area and low equipment costs. Here, through a unique design of the fabrication and transferring processes, we achieve a facile transferring of wafer-scale ultrathin alumina membranes (UTAMs) onto substrates without any twisting, folding, cracking and contamination. The most important in our method is fixing the UTAM onto the wafer-scale substrate before removing the backside Al and alumina barrier layer. It is also demonstrated that the thickness and surface smoothing of UTAMs play crucial roles in this transferring process. By using these perfectly transferred UTAMs as masks, various nanostructure patterning including nanoparticle, nanopore (nanomesh) and nanowire arrays are fabricated on wafer-scale substrates with tunable and uniform dimension. Because there are no requirements for UTAMs, substrates and materials to be deposited, the method presented here shall provide a cost-effective platform for the fabrication of ordered nanostructures on large substrates for various applications in nanotechnology.

Sub-100-nm nanoparticle arrays with perfect ordering and tunable and uniform dimensions fabricated by combining nanoimprinting with ultrathin alumina membrane technique.

This work reports a nonlithographic nanopatterning approach to fabricate perfectly ordered nanoparticle arrays with tunable and uniform dimensions from about 30 to 80 nm and strict periods of 100 nm in a square lattice on large-area substrates by combining nanoimprinting with ultrathin alumina membrane technique. There is no requirement of any organic layer to support an ultrathin membrane in our novel route, which totally addressed the problems of nonuniform pores in prepatterned alumina templates and contamination during sample preparation, and thus is indispensable for our fabrication of ideally regular nanoparticle arrays on various kinds of substrates (such as flexible plastic). The effect of imprinted pressure on the prepatterning of Al foil was also studied in order to ensure the reusability of the precious imprinting stamps. This simple but efficient method provides a cost-effective platform for the fabrication of perfectly ordered nanostructures on substrates for various applications in nanotechnology.

Study of interface electric field affecting the photocatalysis of ZnO.

Based on a novel Al-doped ZnO thin film/ZnO single crystal system, the effects of interface electric field on photocatalysis were investigated in detail. Both the position and intensity of the electric field are found to have decisive influence on the photocatalytic activities.